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Two photos, released by NASA on Thursday, show a hole designated "Ding 18" on a radiator panel on the shuttle Atlantis. The inset photo is a closeup of the area indicated by orange label in the overall photo of the radiator panel. "Ding 18" looks like a bullet hole beside the orange label.

Space shuttle experts are examining a small hole that was discovered on Atlantis after it landed — damage apparently done by a piece of space debris during the shuttle's flight.

The hole, labeled "Ding 18," is about a tenth of an inch (2.7 mm) in diameter and goes all the way through a quarter-inch (6 mm) aluminum honeycomb panel.

It was spotted on the surface of the aft-most segment of the starboard shuttle thermal radiator, one of two panels that unfold from the spacecraft's open payload bay doors in flight to dump excess heat from the cabin. The object missed the nearest pressurized coolant line by several inches.

The brief notice of the hole’s discovery appeared in the NASA minutes of a regular meeting at Kennedy Space Center in Florida, called the “Integrated Flow Status,” where the processing of all space shuttle components is discussed. The meeting is chaired by Robert Lightfoot, the manager of the Space Shuttle Propulsion Office at NASA's Marshall Space Flight Center in Huntsville, Ala., and is attended by both NASA and contractor specialists and managers.

The report came to light Wednesday on the independent NASASpaceFlight.com Web site, which posted an excerpt calling the hole the “first or second largest hit" in the history of the space shuttle program. Managing editor Chris Bergin, said the report was updated Wednesday and acquired from an unnamed source.

Another Web site, CollectSpace, posted a photograph showing the damage on Wednesday, and by Thursday NASA released the photos and schematics on its own Web site. MSNBC.com's efforts to obtain public comment from NASA on the debris strike have so far been unsuccessful, but sources discussed the damage and the follow-up inspections on condition of anonymity.

Because the impacting object apparently was vaporized, inspectors are now collecting samples from surrounding surfaces to try to figure out what the object was. In the past, residue has sometimes identified smaller impactors as being either human-made debris (such as paint chips from satellites) or natural micrometeoroids.

The debris damage comes only three weeks after the in space station crew noticed "a small hole" in one of the Russian segment's solar panels. A Sept. 14 internal status report said that the crew members downlinked photos of the damaged area, taken from a window in the station's Zvezda service module.

Russian Mission Control told NASA officials that four electricity-generating cells were affected, but power output remains at expected levels. Such impacts occur from time to time, but are usually much smaller. Photos of the solar-cell damage have not been publicly released.

Because Atlantis' "ding" is on a surface that was not exposed during the shuttle's fiery descent to Earth, the crew was not in danger. However, had the radiator been crippled, a mission emergency could have been declared and the shuttle would have had to land within a day, potentially aborting the station assembly mission.

Vigilant about damage
NASA managers have been much more vigilant about debris damage in the wake of the Columbia tragedy in 2003, which led to the loss of that space shuttle and all seven astronauts aboard.

NASA

A cross-section diagram of the damage, released by NASA on Thursday, showed that behind the tenth-inch entry hole was a blown-out cavity 10 times as wide. The impact fell fortuitously right between two small pipes (shown in blue) carrying pressurized coolant through the radiator.

Shortly after Columbia's launch, a large piece of fuel tank insulation hit the leading edge of Columbia's left wing, smashing a head-sized hole in the wing's reinforced carbon-carbon panel, or RCC. That allowed superheated gases to enter the wing during the shuttle's atmospheric re-entry, destroying Columbia from within.

The daily reports from Mission Control to the crew didn't contain any reference to the damage, raising the possibility that it escaped detection during the numerous camera sweeps that were performed in flight. Those examinations focused on the shuttle’s thermal protection system — its heat-resistant tiles, blankets and reinforced wing panels — rather than on the radiator panels.

During the preparations for Atlantis' landing, NASA spotted what appeared to be several small objects floating near the shuttle — leading to an extra camera survey of the heat shield. The suspicion was that they were pieces of the shuttle knocked loose by debris impact, or jarred loose by thruster test firings.

That extra survey, like the mission's previous inspections, did not focus on the radiator panels.

Missed opportunity in 2003?
The extra checks were made in an atmosphere haunted by a tragic missed opportunity leading up to the Columbia catastrophe. Investigators say a piece of Columbia's broken heat shield panel shook loose during some thruster firings on the day after launch and drifted off into space. For many minutes, the debris was well within range of the shuttle’s cameras and the crew members’ eyeballs — but nobody noticed.

This was realized weeks after tragedy struck, when U.S. Air Force space radar trackers reviewed the raw data collected — but not evaluated — during the mission. The piece was tracked by sensors around the world, at such precision that its shape and mass could be estimated. It matched a broken-off, curved panel with supporting ribs.

Had the object been seen, many flight controllers now feel, enough suspicion would have been raised to look more closely for heat shield damage. The hole in the wing might have been found, and NASA might have been alerted to the threat.

Perhaps some emergency repair or rescue could have been pulled off, or perhaps not — post-disaster analysis reached no firm conclusions on that score. But in any case, the shuttle crew and their support staff at Mission Control would have gone down swinging, rather than being blindsided by disaster.

What ifs for the new damage
Shuttle specialists familiar with the latest "ding" on Atlantis told MSNBC.com that it was difficult to say for sure how bad the damage might have been if the impacting object had hit elsewhere on the shuttle. These specialists spoke on condition of anonymity because they were not authorized to discuss the situation publicly.

"First assessments were if it had hit RCC it would have penetrated but would be within the allowable damage criteria," one said in an e-mail. Another source suggested, however, that an oblique or glancing impact could have wreaked significant damage and required a repair attempt.

Over the course of the shuttle program, NASA has developed hardware and software measures to guard against a scenario in which an impact penetrates a coolant line on the radiator and leads to the loss of that critical system. Extra shielding has been welded over the lines, and the shuttle's computers monitor the lines for any pressure drop, ready to shut off isolation valves to prevent the loss of coolant fluid.

The shuttle can also be cooled by expelling water through a system called the flash evaporator — which is meant to operate at the beginning and end of each flight, when the payload bay doors are closed. The apparatus could be turned on to supplement a degraded radiator panel system, but that would force the shuttle to return to Earth at the earliest opportunity.

The specialists agreed that the least vulnerable structures on the shuttle would include its triple-paned windows. Any small debris hitting the outer panel would vaporize, and if debris penetrated to the second layer, it would be in the form of a fast-moving dust cloud with much less punch than the original solid object.

The 12 orbiting astronauts on the International Space Station were hailed by the chief on Thursday — and also got a humorous scolding for keeping their android crewmate locked up in its packing for so long.